1. Field of the Invention
The present invention relates to a digital terrestrial TV (television) broadcast signal receiving system which uses a multi-directional antenna called smart antenna and a digital terrestrial TV broadcast signal receiver suitable for the system. Especially, the present invention relates to digital terrestrial TV broadcast signal receiving system and receiver capable of remote control of gain of an amplifier built in the multi-directional antenna. In this description, the digital terrestrial TV broadcast signal receiving system and receiver are abbreviated as TV broadcast signal receiving system and TV broadcast signal receiver as appropriate.
2. Description of the Related Art
In the North American Continent where an ATSC digital television broadcast (it is abbreviated to a hereinafter digital TV broadcast) is performed, plains are dotted with metropolises, so that various digital TV broadcast signals broadcasted in the suburbs of each metropolis can be received at other cities or the halfway point of the cities. In consideration from a user who receives the TV broadcast signals, since many TV broadcast signals are transmitted in various directions, it is necessary to regulate a direction of an antenna in an orientation to a TV broadcasting station from which a program which he wants to watch is delivered. Therefore, a multi-directional antenna such as a smart antenna is put into practical use.
The digital terrestrial TV broadcast has a characteristic that when an intensity of received signal of a digital TV broadcast signal is equal to or larger than a predetermined threshold value, an image having a predetermined image quality can be obtained by correction. Thus, it is possible to watch TV programs of various channels with using a multi-directional antenna.
Generally, the multi-directional antenna is installed in an outdoor space, a TV broadcast signal receiver having functions of a tuner and a decoder is disposed in an indoor, and the multi-directional antenna and the TV broadcast signal receiver are connected by cables. The level of the TV broadcast signal outputted from the multi-directional antenna is varied from low level near to a threshold to very high level corresponding to signal receiving condition. Therefore, the TV broadcast signal receiver adjusts a gain of an amplifier for amplifying the TV broadcast signal corresponding to the level of the TV broadcast signal outputted from the multi-directional antenna. Although the amplifier is provided on the multi-directional antenna, the gain of the amplifier is fixed in the multi-directional antenna marketed at present.
Since the multi-directional antenna and the TV broadcast signal receiver are connected by the cables, it is inevitable that noise is superimposed on the TV broadcast signal transmitted through the cable. Therefore, when the level of the TV broadcast signal is lower, the S/N, which is originally lower, is further reduced by the noise superimposed on the TV broadcast signal while it is transmitted through the cables. Even if the TV broadcast signal that the S/N is reduced is amplified and performed a signal correction process such as filtering in the TV broadcast signal receiver, the noise may not be removed sufficiently so that quality of an image displayed on a screen of a display apparatus may be deteriorated. Furthermore, the level of the noise superimposed on the TV broadcast signal is varied corresponding to the locations of the multi-directional antenna and the TV broadcast signal receiver, the lengths of the cables, and so on. Therefore, even when the same combination of the multi-directional antenna and the TV broadcast signal receiver is used, the signal receiving condition of a specific cannel becomes unstable corresponding to the installation condition of the multi-directional antenna, so that the TV programs of the specific channel may not be watched.
On the other hand, Japanese Laid-Open Patent Publication No. 11-289499 discloses a TV receiver having two tuners in which a gain of a distributor for distributing TV broadcast signals into two ways is adjusted in optimum state with respect to each channel. A difference between levels of TV broadcast signals of a reception channel and an adjoining cannel. When the level of the TV broadcast signal of the adjoining channel is higher than that of the reception channel, there may be a possibility that the TV broadcast signal of the reception channel is intercepted by the TV broadcast signal of the adjoining channel. Therefore, the gain of the distributor is set to be lower. Alternatively, when the level of the TV broadcast signal of the adjoining channel is lower than that of the reception channel, the gain of the distributor is set to be higher so as to increase the S/N.
Japanese Laid-Open Patent Publication No. 9-93509 discloses a TV broadcast signal receiving apparatus for mobile such as an automobile having a high frequency amplifying circuit and an intermediate frequency of amplifying circuit capable of gain adjustment, and a means for correcting frequency characteristic of the intermediate frequency amplifying circuit for improving multipath disturbance caused by reflected wave from a structure such as a building. When the frequency characteristic of the intermediate frequency amplifying circuit is corrected, a start point of the gain control of the high frequency amplifying circuit is shifted to a side where a level of an input signal is higher.
Japanese Laid-Open Patent Publication No. 2004-193723 discloses an intermediate frequency processor which can receive a TV broadcast signal and an FM radio broadcast signal. When the FM radio broadcast signal is received, an operation of a VIF amplifier is switched off by a control signal from outside.
Japanese Laid-Open Patent Publication No. 57-42218 discloses a delayed automatic gain control circuit comprising a first automatic gain control circuit and a second automatic gain control circuit which operates when an output of the first automatic gain control circuit exceeds a predetermined threshold value. The second automatic gain control circuit is configured by a circuit for detecting a peak value by taking out a difference between a peak value of the output of the first automatic gain control circuit and the threshold vale, so that variation and reduction of a loop gain is prevented.
By the way, it is an inherent problem of the digital terrestrial TV broadcast that the signal receiving condition becomes unstable due to the locations of the multi-directional antenna and the TV broadcast signal receiver and the lengths of the cables, and thereby the TV program of a specific channel may not be watched. Therefore, the problem cannot be solved by applying the conventional techniques described in the above-mentioned documents.